The TDM/TDMA principle is well known in radio systems or passive optical networks (PONs), where it is employed to permit transmission between a single base station and a plurality of outstations. In the downstream (base station to outstation) direction, the information (traffic) is broadcast to all outstations, but upstream it is transmitted in bursts, each of which must be timed to avoid mutual interference (overlap) so that at any time the base station only receives data from one outstation. When a new outstation is to be connected its time of transmission must be such that it does not interfere with existing traffic transmissions and the processing required to ensure this is referred to as marshalling.
A known method of marshalling involves reservation of a period of no transmission in the upstream frame (a window). It will be appreciated that the base station broadcasts information to all outstations employing a frame, which is typically of length 125 microseconds. The outstations are all able to receive the downstream traffic at a different time, depending on their distance from the base station and the propagation delay.
Typically, in the steady state, several outstations may be already configured to send their data burst to the base station at a time which will ensure their arrival in separate bursts. A new outstation will, according to the known marshalling method, make a trial transmission into the window. The position of this is subsequently detected at the base station and used to determine the marshalling delay. In this known approach, the window in the upstream traffic flow must be at least as large as the uncertainty in the downstream and upstream loop delay. For example, with a PON, the loop delay may be 200 microseconds for a 20 Km range (i.e. the outstations may be up to 20 Km from the base station), requiring a 200 microsecond window, which would result in an equivalent additional delay in the upstream traffic, the need for all operating outstations to queue for one window's duration traffic and necessitates subsequent storage facilities, and a loss of bandwidth given by the product of the frequency of the window and its size in bits.
Our published United Kingdom specification No.2272610 describes a method of marshalling an outstation of a TDMA telecommunications system, the system including a base station and a plurality of outstations. The method includes transmitting from the outstation a sequence at a level below the noise sensitivity of a receiver at the base station, detecting the sequence at the base station, discriminating the phase of the detected sequence, and using the discriminated phase to determine the loop delay to the outstation. The sequence and its phase are determined by a correlation process.
It is an object of the present invention to provide an improved marshalling process.